Some of the brightest objects in the Universe are active galactic nuclei: powerful jets of light and matter powered by a galaxy's central black hole. The interaction between these jets and the galaxy's gas is known as galactic feedback and is thought to have a strong impact on star formation. However, this interaction occurs very close to the center of a galaxy, which is a dense region that's difficult to observe at a high resolution.

Difficult, but not impossible. Raffaella Morganti, Judit Fogasy, Zsolt Paragi, Tom Oosterloo, and Monica Orienti used a global network of radio telescopes to observe the core of a distant bright galaxy, one that harbors a black hole that recently became active. The team located the position where the jet from the black hole begins to mix with the galaxy's hydrogen gas, driving it away from the central regions of the galaxy. This measurement provides an excellent test of models of galactic feedback, opening a new way of understanding how black holes shape the environments far from where their gravity dominates.

Active galactic nuclei (AGNs) are powerful gravitational engines, channeling plasma—hot ionized gas—into disks and jets. We observe these as quasars and other bright objects, visible across billions of light-years. However, not every galaxy harbors an active black hole; the Milky Way is a prime example of an inactive galaxy, though it shows signs of past activity.

That's important: galaxies are not static entities, and their individual histories can include periods of intense star formation as well as an AGN phase. Young stars and active black holes can both make a galaxy luminous, and sometimes they are present at the same time in the same galaxy. One example of this category carries the catalog designation 4C12.50. It's also what's called an ultraluminous infrared galaxy (ULIRG), meaning that most of its emitted light is in the infrared. These galaxies provide an important laboratory for understanding the coupling between the jets of the central black hole and the complex gas dynamics that drive star formation.

According to theoretical models, the plasma driven out in jets from an AGN hits colder, neutral gas that is present beyond a galaxy's central region. Even though the jet itself is narrow, when it strikes a lumpy cloud of gas, the disturbance propagates outward, possibly driving a wide area of gas farther away from the galactic center. The effect of this galactic feedback can stifle star formation near the region of interaction but increase it farther out.

Previous observations had identified ouflows of neutral hydrogen in 4C12.50, but they lacked the resolution to see whether they were driven by the AGN jets. The present study solved the problem by using very long baseline interferometry VLBI, a technique that chains widely separated radio telescopes together into a network. In this case, the observers used the Very Long Baseline Array (VLBA), stretching across the United States and its territories in tandem with the European VLBI Network. Very long baseline interferometry combines the data from each telescope to create a far higher resolution than any single instrument could achieve.

The result was striking: the researchers imaged the central region of 4C12.50 at sufficient precision to pinpoint where the AGN jets hit the colder hydrogen gas. That point is approximately 300 light-years from the center of the galaxy (roughly half the distance between Earth and Betelgeuse). When the powerful, radio-emitting plasma struck the hydrogen, it created a large plume flowing in the same direction. The result was a lobe of warmer gas being expelled from the galaxy's central region at 1,000 kilometers per second (2 million miles per hour).

The association between the AGN jet and the previously measured outflow of gas in 4C12.50 is a clear detection of galactic feedback. Since AGNs change in their output over their lifetime, waking up and quieting down as they feed, the feedback will also fluctuate over time. That in turn should have a profound impact on the temperature of the galaxy as a whole, as well as the rate of star formation.